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KINETICS AND MECHANISM OF HYDROLYSIS OF TRI 4 CHLOROTHIOPHENYL PHOSPHATE ESTER VIA CONJUGATE ACID SPECIES

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KINETICS AND MECHANISM OF HYDROLYSIS OF

TRI-4-CHLOROTHIOPHENYL PHOSPHATE ESTER VIA CONJUGATE

ACID SPECIES

Asha Verma1 and Firdous Andleep2*

1

Professor of Chemistry, 2Research Scholar

Department of Chemistry, Govt. Science and Commerce College, Benazir, Bhopal, India.

ABSTRACT

Hydrolysis of Tri-4-chlorothiophenyl phosphate ester has been carried

out in the region of 0.1-7.0 mol dm-3 HCl at 980C. Acid log rate profile

has a maximum at 4.0 mol dm-3 HCl and then decreases after 4.0 mol

dm-3 HCl. This decrease in rate is attributed to water activity and

negative effect of ionic strength. The theoretical and observed rates

estimated from second empirical term of Debye- Huckle equation have

been found to be in close agreement. Bimolecularity of the reaction has

been determined from Arrhenius parameters and molecularity data.

The triester involves P – S bond fission which is strengthened by

comparative kinetic data.

KEYWORDS: Tri-4-chlorothiophenyl phosphate, bimolecularity, trimester, Arrhenius

parameter.

INTRODUCTION

Organophosphates having C-S-P linkage are important class of compounds that find their

applications in many fields. Besides their antiviral activity[1] and radioactive tracer

techniques[2], they are used in biological investigations, insecticidal activity[3] and textile

commodities.[4] Taking this in view Tri-4-chlorothiophenyl phosphate was chosen for kinetic

study as this compound is reactive via its different reactive species, depending upon the

experimental conditions.

Volume 6, Issue 3, 1247-1252. Research Article ISSN 2277– 7105

*Corresponding Author

Firdous Andleep

Research Scholar,

Department of Chemistry,

Govt. Science and

Commerce College, Benazir,

Bhopal, India.

Article Received on 13 Jan. 2017,

Revised on 02 Feb. 2017, Accepted on 23 Feb. 2017

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EXPERIMENTAL

Tri-4-chlorothiophenyl phosphate was prepared by using PCl5 as phosphorylating agent[5,6,7]

The ratio of 3:1 thiol and PCl5 was employed for this preparation.

Theoretical: C = 45.244, H = 2.5314, Cl = 22.2589, P = 6.4835, S = 20.132, O = 3.3485

Observed: C = 45.11, H = 2.325, Cl = 21.908, P = 6. 246, S = 20.03, O = 3.186.

The kinetic analysis of the present triester was carried out by hydrolyzing it in the range of

0.1-7.0 mol dm-3 HCl and 1.24 – 7.46 pH in aqueous 10% dioxan-water medium (V/V) at

980C. The constant ionic strength was maintained using appropriate mixtures of HCl and

NaCl.

RESULTS AND DISCUSSION

From the hydrolysis of present triester, pseudo first order rate coefficients are found to

increase with the increase in acid molarity upto 4.0 mol dm-3 HCl in the range of 0.1-7.0 mol

dm-3 HCl. Further increase in acid molarity after 4.0 mol decreases the rate constant due to

negative effect of ionic strength. Hydrolysis at three different ionic strengths i.e.,1.0, 2.0 and

3.0µ is denoted by linear curve that makes a negative slope with acid axis indicating the

presence of acid catalysis (slope values KH+ = 47.77, 31.57 and 25.91 for1.0, 2.0 and 3.0µ

respectively. Each of these lines may be represented by

Ke = KH+. CH+ ……….. (I)

Where

Ke, KH+ and CH+ are experimental acid catalysed and specific acid catalysed at that ionic

strength and and hydrogen ion concentration respectively.

Figure 1: Acid catalyzed hydrolysis of Tri-4-chlorothiophenyl Phosphate at constant

[image:2.595.134.464.573.721.2]
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From the study of ionic strength effect, the total rate contributed by conjugate acid species

and neutral species can be calculated by second empirical term of Debye- Huckle equation.[7]

Ke = KH+. CH+ + KN ………….. (II)

Table 1: Estimated and experimental rates of hydrolysis of Tri-4-chlorothiophenyl

phosphate at 98c

HCl mol dm-3

KN ×

103 min-1

KH+CH+

× 103 min-1

Ke×103

min-1 (aH2O)

n Ke×10 3

min-1 Estm.

Ke×103

min-1 Expt.

3+log Ke

Estm.

3+log Ke

Expt.

0.1 18.0 6.071 24.071 24.071 24.01 1.381 1.380

0.5 18.0 26.8 44.85 44.85 44.63 1.652 1.649

1.0 18.0 46.13 64.13 64.13 64.25 1.807 1.808

1.5 18.0 59.36 77.36 77.36 76.98 1.888 1.886

2.0 18.0 67.92 85.92 85.92 85.56 1.934 1.932

2.5 18.0 72.69 90.69 90.69 90.79 1.957 1.958

3.0 18.0 74.99 92.99 92.99 92.83 1.968 1.967

3.5 18.0 75.08 93.08 93.08 93.00 1.969 1.968

4.0 18.0 77.62 95.62 95.62 95.89 1.981 1.982

5.0 18.0 67.76 85.77 (0.155)1 65.32 68.91 1.815 1.838 6.0 18.0 59.84 77.84 (0.211)2 40.65 42.34 1.609 1.627 7.0 18.0 51.40 69.40 (0.279)3 25.482 26.59 1.406 1.425

From the above table it is observed that estimated and experimental rates agree well with

each other and the decrease in rate after 4.0 molarity of HCl may also be attributed to water

activity along with negative salt effect.

The rate law may be formulated as

(1) In the region from 0.1 to 4.0 mol dm-3HCl

Ke = 62.66 × 10 -3 min-1 CH+. exp. (-0.133 × 2.303). µ + 18.00 × 10 -3min-1

(2) In the region > 4.0 mol dm-3 HCl

Ke = 62.66 × 10 -3 min-1 CH+. exp. (-0.133 × 2.303). µ. (aH2O)n + 18.00 × 10 -3 min

Where (aH2O)n is water activity and n is an integer.

The magnitude of Arrhenius[8] parameters determined for the hydrolysis at 3.0 and 5.0 mol

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[image:4.595.79.522.111.198.2]

Table 2: Arrhenius parameters for the hydrolysis of Tri-4-chlorothiophenyl phosphate

in acid media

HCl mol dm-3

Parameter Energy of

Activation "E" K cal mol-1

Frequency factor "A"

Sec-1

Entropy of Activation - ∆S≠ e.u.

3.0 4.58 2.06×10-3 51.956

5.0 1.35 2.03×10-3 51.551

Bimolecular nature is further supported by Zucker- Hammet9 (0.330), Hammet10 (0.168) and Bunnet (ω = 10.65, ω*

= 4.51) plots. Bunnet – Olsen parameter (ϕ = 1.37) which is greater

than 0.58 suggests that water is involved as proton transfer agent in the rate determining step.

The effect of solvent shows a significant rise in rates due to better proton donating properties

of dioxin.[13] Hence the solvent effect (table not shown) may be taken in accordance with Chanley’s.[13]

observation, indicating the formation of a transition state in which charge is

dispersed. Bimolecular nature is further supported by comparative kinetic data involving P-S

bond fission.

Mechanism

Hydrolysis of Tri-4-chrorothiophenyl phosphate via conjugate acid species, may be

formulated as follows.

(1)Formation of conjugate acid species of triester by fast pre-equilibrium proton

transfer

O

S P S

Fast Cl

S

Cl

Cl Cl + H3+ O H2O

Cl

O

S P S

S

Cl H

Cl +

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(2)Bimolecular heterolysis of P-S bond of the acid species by SN2 (P)

O

S P S

SLOW Cl

O H

H

H

Cl

O

Cl

S

Cl H

Cl

Transition State S...P S

H

H

δ δ

O

Cl S

Cl SH + O P S

O

S H

H

Cl

Cl

Fast proton transfer

O

S

HO Cl

Cl

P S + H

Fast

Diester

This is followed by the hydrolysis of diester into monoester and then into inorganic

phosphate.

CONCLUSION

New research in the field of kinetics of phosphate esters can help an academician to design an

orthophosphate pesticide with low toxicity and discovery of novel bioactive molecules. So

that they should be more specific in their action and instrumental in raising crop yields with

high rates of degradation by soil bacteria of several genera and decompose eventually. Also

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ACKNOWLEDGEMENT

The authors are thankful to the Dept. of Chemistry, Govt. Science and Commerce College,

Benazir, Bhopal for providing the required lab facilities for carrying out the research work.

REFERENCES

1. Synthesis and antiviral activity, (1961), Chem. Abstr., 54: 1303.

2. Gardner, G., & Kilby, B.A., (1950), J. Chem. Soc., 3: 1769.

3. Schlesinger, A. H., (1955), Chem. Abstr., 49: 5517.

4. Dayer, H. N., (1959), Chem. Abstr., 53: 1772.

5. John, A.D., (1923), St., U.S. Pat. Z., 462: 306.

6. Shuman, R.L., (1938), U.S. Pat. 2, 133: 310

7. Williamson, Ann., 1854; 92: 316.

8. Arrhenius, S. Z., (1889), Phys. Chem., 4: 226.

9. Zucker, L. and Hammet, P. , (1932), J. Am. Chem. Soc., 61: 2791.

10.Hammet, L.P., (1940), Physical Organic Chemistry, McGraw-Hill, London, 335.

11.Bunnet, J.F., (1961), J. AM. Chem. Soc., 83: 4956.

12.Bunnet J.F. and Olsen, F.P., (1966), Can. J. Chem., 44: 1917.

Figure

Figure 1: Acid catalyzed hydrolysis of Tri-4-chlorothiophenyl Phosphate at constant Ionic strength at 98⁰c
Table 2: Arrhenius parameters for the hydrolysis of Tri-4-chlorothiophenyl phosphate

References

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